Safety helmets such as are used in industrial safety helmets or as crash helmets for motorcyclists consist in essence, of a hard outer shell of metal or rigid plastic with an insert of semi-rigid foamed plastic such as polystyrene foam and glued to the inside of the foam insert, a lining of leather, flexible plastic, or textile material. This flexible inner lining is commonly backed with a layer of soft foam padding some 5-12mm thick made, for example, of foam rubber.
For economic reasons, such safety helmets are manufactured in only a limited number of head sizes, namely so-called "double sizes", i.e., sizes intended to fit two adjacent head sizes. In order to improve the fit of such industrial safety helmets, which is naturally only very approximate, on the head of the wearer, consideration has already been given to the possibility of attaching, beneath the foam-padded lining of the industrial safety helmet so that it extends over the helmet's inner surface, an inflatable rubber or plastic cushion, the variable degree of inflation of which is intended to produce the desired adjustment to the head size.
This adjustment of the fit may in general be sufficient in the case of an industrial safety helmet. However, the adjustment is still not ideal for motorcycle helmet construction, since in the case of motorcycle helmets, the requirements regarding precision of fit are considerably higher than for industrial safety helmets.
With motorcycle helmets, it is far more important than with industrial safety helmets that they should be not only easy to put on but also and above all, easy to take off, particularly with regard to the possibility of accidents involving injury to the head or neck. This requirement could be met comparatively easily by selecting a relatively loose fit. However, since it is very important that motorcycle helmets, unlike industrial safety helmets, should also sit firmly on the wearer's head in order reliably to prevent their slipping from the correct position as a result of wind pressure especially at high speeds, this solution cannot be used here. For the same reason, merely adjusting the helmet to the wearer's head size is not sufficient for a really ideal fit in this case either. What would be desirable, in addition to this, is the closest possible individual adjustment of the internal shape of the helmet to the various head shapes of the wearers which, of course, comprise a much larger number of possible variations than mere head sizes, something which is not possible with helmets of the kind known hitherto.
In the case of the best of the motorcycle helmets known today, the so-called integral helmets, it is of crucial importance that these requirements should be met. Integral helmets cover not only the wearer's cranium and temporal regions but also the entire head including the face, leaving only an aperture in front of the face which is expediently closed by a transparent windscreen. Since such helmets must closely surround all parts of the head including the chin they are not at all easy to remove especially for unskilled laymen which can cause considerable problems in the event of injury to the wearer.